[S-CG58] New Developments in fluid-rock Interactions: From Surface to Deep Subduction Zone

[SCG58-P02] Syenitoids from Toubyohana, northern Hakata Island, Ehime Prefecture, Japan: a geological record of metasomatism of granitoids induced by two distinct hydrothermal fluids

*Toko Fukui¹, Satoshi SAITO¹, Toshiro Takahashi² (1.Graduate School of Science and Engineering, Ehime University, 2.Department of Geology, Faculty of Science, Niigata University)

Keywords:Hakata Island, Syenite, metasomatic alteration

In this study, we describe field occurrences, petrographic remarks and geochemical characteristics of metasomatic syenitoids occurring in the Cretaceous granitoids from Hakata Island, Ehime Prefecture, Japan. Two types of syenitoids (Type 1 and Type 2) are distinguished based on field and petrographic observations together with geochemical characteristics. Chemical changes during metasomatism are estimated from the isocon diagram assuming the Ce to be immobile element. Compared to the host granitoids, Type 1 syenitoids are characterized by increases of Ti, Mg, Mn, P, Be, Co and Cd, and decreases of Si, B, W and Pb, whereas Type 2 syenitoids are characterized by increases of Na, K, Li, Rb, Nb and Ta, and decreases of Ca, Cs, Pb and Eu. The contrasting chemical characteristics observed in the syenitoids imply that two distinct hydrothermal fluids with different chemical compositions have induced metasomatism of Cretaceous granitoids in the studied area.